Each year a number of orphan and triplet lambs have to be reared artificially. The most common method for rearing these animals, in large flocks, is to place them in groups on milk bars without any maternal care. Not only is this procedure costly and labor intensive, but it also deprives the lambs of maternal care which can lead to the development of social and sexual abnormalites in adulthood. Further, animals raised on milk bars cannot derive the full immunization benefit which would normally follow from the antibodies passed on to them through a ewe's milk. It is therefore clearly preferable from a welfare, health and economic standpoint to raise orphan and triplet lambs with proper foster mothers. During the past 7 years we have attempted to establish how lactation and maternal behavior are stimulated in the sheep, and what controls the formation of the selective recognition bond between the ewe and its lambs. The primary aims of this work have been to establish reliable protocols for (a) Fostering orphan lambs onto maternal ewes, to avoid the necessity of rearing them without maternal care on milk bars and (b) To improve poor quality maternal care, particularly in primiparous animals, which also contributes to increased mortality and suffering in lambs. The main approach adopted to provide these protocols has been to investigate the importance of the various physiological changes occurring in sheep during pregnancy, parturition and post-partum for the control of lactation and maternal behavior.
A series of studies has established that in sheep, the primary stimulus for inducing maternal responses towards new born lambs is feedback from the vagina and cervix reaching the brain. Thus, maternal behavior can be induced in non-pregnant sheep simply by mechanically stimulating the vagina and cervix. Conversely, blocking the signals from the vagina and cervix reaching the brain, using epidural anaesthesia, prevents ewes from showing normal post-partum maternal behavior.
The changes in blood concentrations of progesterone and oestradiol which occur during pregnancy are essential for the production of lactation and for the ability of stimulation of the vagina and cervix to induce maternal behavior. These steroids are not however, as had previously been proposed, the major factors controlling the stimulation of this behavior. Maternal experience also plays a role, and we have found that nulliparous ewes are less responsive to the effects of stimulation of the vagina and cervix, following short term treatment with progesterone and/or oestradiol, than multiparous ones, and this may partly explain why primiparous ewes are often poor mothers.
Stimulation of the vagina and cervix during parturition is also important for the formation of the selective olfactory recognition bond between a ewe and its lambs and we have found that artificial mechanical stimulation of these structures can reliably induce ewes to accept and form new recognition bonds with orphan lambs even 24 h post-partum. Importantly, this stimulation of the vagina and cervix does not interfere in any way with the recognition bond already formed with the ewes own lambs and works equally well in maternal multiparous or primiparous ewes.
In a series of studies where we have sampled changes in brain chemistry, which might be associated with the post-partum stimulation of maternal behavior, we have shown that the peptide hormone oxytocin is released in the brain during parturition and following artificial vaginocervical stimulation. This release does not occur when the animals do not show maternal behavior following an epidural anaesthetic block. Furthermore, infusion of this peptide into the brain ventricles stimulates maternal behavior in non-pregnant ewes within 30 s, provided that they are pre-treated with oestradiol. The source of oxytocin within the brain is primarily derived from cells in the brain which do not project to the posterior pituitary. The blood-brain barrier is relatively impermeable to the peptide so that peripheral injections of it, or the high concentrations occurring in the blood at parturition, cannot enter into the brain to exert any behavioral action. We have shown that the oxytocin-containing cells in the sheep brain express oxytocin mRNA and that the levels of mRNA expression are at their highest during parturition and lactatation, when maternal behavior is displayed. Oxytocin mRNA levels and release of oxytocin within the brain are increased following exogenous treatment with progesterone and/or oestradiol, further underlying the important modulatory role that these steroids have on the brain oxytocinergic cells controlling the induction of maternal behavior. We have also found evidence that endogenous opiate pathways in the brain influence both the ability of artificial stimulation of the vagina and cervix to induce maternal behavior and central oxytocin release.
It has been known for many years that the formation of the selective recognition bond between ewes and lambs is olfactory, since rendering ewes anosmic (by removing the olfactory bulbs in the brain) prevents it and they will therefore mother any lambs. Research suggests that noradrenergic and possibly cholinergic pathways are important for the formation of the selective recognition bond. We have recently shown that noradrenaline and acetylcholine are released in the olfactory bulbs during parturition and during induction of fostering of orphan lambs using artificial stimulation of the vagina and cervix. After the establishment of the selective bond, these transmitters are also released following brief exposure to lamb odor. These changes do not occur immediately at parturition in primiparous animals, but after a delay of several hours and this may explain why these animals take longer to form the selective recognition bond with their lambs than multiparous ewes.
Our extensive behavioral and physiological studies have therefore clearly established the importance of the influence of stimulation of the vagina and cervix for mediating profound changes in brain chemistry which alter the behavior of ewes towards lambs. While administration of artificial stimulation of the vagina and cervix is extremely effective in inducing ewes with single lambs to adopt orphan lambs up to 24 h post-partum, this does not provide a total solution for fostering all orphan or triplet lambs since the availability of suitable mothers for fostering is extremely unpredictable. As an alternative solution we have surprisingly found that non-pregnant animals could act as foster mothers. For this approach to be a viable proposition, it is essential that lactation be artificially induced simply, quickly and cheaply; and it is also essential to induce permanent high quality maternal care towards lambs so that they are reared successfully to weaning.
Previous studies inducing lactation in sheep have relied on protocols where steroid hormones or other substances are injected into the animals over long periods. Such procedures are neither practical nor economical for farming. We instead developed a protocol based on topical application of at least one steroid hormone, preferably a combination of a progestogen and an oestrogen such as a combination of progesterone and oestradiol. These hormones are also normally present in high concentrations in the blood during late pregnancy. It is known in current farming practice to use vaginal sponges impregnated steroid for synchronizing oestrous in animals. It is also known (S. R. Davis et al, 1983. J. Dairy Sci. 66:450-457) that it is possible to induce lactation of non-pregnant cows by use of an intravaginal sponge impregnated with 500 mg estradiol-17.beta. and 1000 mg progesterone inserted for 10 days and then removed. Treatment with the sponge for 10 days resulted in lactogenesis in 25% of treated cows. Udders of cows induced to lactation by intravaginal sponge (IVS) treatment began to fill with secretion 8 to 10 days after IVS insertion and milking was begun 12 days after insertion (2 days after IVS removal).